You are asked to add ballast to a helium-filled ballon until it will float in the centre of the room?

Question

A) What could you use as ballast?
B) What happens if you add or remove some of this ballast?

Answer 1

You could tie a long string or ribbon on it and let it go. At some point, the balloon will stop rising. Cut the string just where it touches the floor. Neutral bouyancy achieved!

Answer 2

This is just a simple bouyancy problem. Air is the "liquid" and the volume of the balloon is the submerged solid.

A ballast could be as simple as a valve in the balloon that changes the volume of the balloon, thus changing the amount of "displaced liquid" and static equilibrium height.


Or you could just add weight to the balloon....